1. Field of the Invention
The present invention relates to a protection system for use with a battery charger and more particularly to a protection system for a battery charger for detecting a thermal runaway condition during charging of a battery and protecting the battery when such a thermal runaway condition is detected.
2. Description of the Prior Art
Various battery chargers are known in the art. Some known battery chargers include protection against a thermal runaway condition. Thermal runaway can cause overheating of a battery during charging which can result in damage to the battery as well as personal injury. Thermal runaway occurs as a result of heating of the electrical conductors in the battery during charging. More particularly, thermal runaway occurs when the electrolyte in the battery gets hot. When the electrolyte gets hot, heat is transferred from the electrolyte to the electrical conductors within the battery causing the temperature of those conductors to rise. Since, the resistance of an electrical conductor is inversely proportional to the temperature of the conductor, I the resistance of the electrical conductor decreases. Thus, if the battery charger is providing a constant voltage to the battery under charge, the current will increase in proportion to the decrease in resistance. During such a condition, the battery voltage does not increase even though the charging current is increased. The battery voltage is known to either remain the same or decrease. The increased charging current causes the electrolyte to get even hotter. resulting in the even more charging current to be drawn by the battery placing the battery in a thermal runaway condition. The risk of a battery experiencing a thermal runaway condition is increased during relatively high ambient temperature conditions.
The effect on the battery as a result of a thermal runaway condition depends on the type of battery. For example, in flooded electrolyte batteries, such as lead acid batteries, a thermal runaway condition can cause the electrolyte to boil off; thereby disabling the battery. In sealed batteries, a thermal runaway condition can cause expansion of the electrolyte and an explosion of the battery and possibly personal injury.
Various hardware solutions have been proposed to solve the thermal runaway problem of a battery. Examples of such hardware solutions are disclosed in U.S. Pat. Nos. 5,214,370 and 6,020,721 as well as US Patent Application Publication No. US 2007/0018780 A1 and International Patent Application Publication No. WO 2004/097956 A2.
These hardware solutions all rely on temperature sensors to detect the battery temperature and the ambient temperature. The temperature sensors for detecting the temperature of the battery are known to be connected to various locations on a battery, such as the battery terminals, the plates in the battery or in the electrolyte. The temperature sensors for sensing the ambient temperature are disposed within the battery charger. By detecting the battery temperature during charging, the battery charger can detect when a battery is experiencing a thermal runaway condition.
Although such battery chargers which rely on external temperature sensors to detect and protect against a thermal runaway condition, there are several problems with such systems. In particular, many conventional batteries are not known to be provided with temperature sensors. As such, temperature sensors must be provided along with the battery charger and connected to the battery. These temperature sensors are typically configured to be connected to a surface of a battery or to a battery terminal to provide an indication of the battery temperature during charging.
There are several known problems with such hardware solutions. For example, in automotive and marine battery charging applications, battery temperature sensors are most often not provided with the battery. Thus, the use of external temperature sensors must be relied upon. However, the service technicians may not always connect the temperature sensors to the battery surface or battery terminals. Without the battery temperature, the battery charging system is unable to detect a thermal runaway condition. Moreover, the electrical conductors attached to the temperature sensors may eventually break off after repeated use.
The '721 patent accounts for such a situation and provides a monitoring circuit for monitoring for broken or disconnected electrical conductors that are connected to the temperature sensors. In the event that a disconnected or broken electrical conductor is detected, the battery charger is placed in a fail safe condition and charging is suspended. Such a solution essentially renders the battery charger unusable until the broken wire is repaired. Moreover, if the thermistor used to sense battery temperature is damaged or becomes disconnected from the electrical conductor or becomes misplaced, the battery charger is rendered inoperable
In order to avoid such problems, some known battery chargers do away with the need for battery and ambient temperature sensors altogether and simply rely on a manual ambient temperature switch on the battery charger and rely on the user selecting an ambient temperature range. For example, such a manual temperature switch is provided on a 15 Amp Smart Battery Charger as manufactured by West Marine of Watsonville, Calif. That battery charger includes a manual ambient temperature selector which allows a technician to manually select three different ambient temperature conditions:                Cold: 32° F.-60° F.        Warm: 60° F.-85° F.        Hot: 85° F.-104° F.        
In such an application, the user selects an ambient temperature range before charging. Although relative high ambient temperatures can result in a thermal runaway condition, the system does not take into account the temperature of the battery and thus is a relatively crude and inaccurate method for detecting a thermal runaway protection. Moreover, the technicians may not always remember to set the manual ambient temperature switch.
Thus there is a need to provide a protection system and method for detecting a thermal runaway condition in a battery and providing protection of the battery when a thermal runaway condition has been detected which does not require the use of external temperature sensors or any actions by a technician or operator.